Investigating amino acid metabolism as a focal point between grapevine biochemistry, berry composition and wine quality determinants

Abstract

Amino acids (AAs) are important compounds that occupy a pivotal position in grapevine metabolism and play a major role in the biological functions of fermentative microbes. Nitrogen status and AAs influence not only general vine health, but also the biochemical composition of the grape berry, which in turn, is the major determinant of wine quality. AAs are important in the wine-making process as they constitute a major source of total fermentable nitrogen for yeast (YAN), impacting fermentation kinetics. AAs have a central role linking primary and secondary metabolism, being the starting precursors for important secondary compounds in grapes including methoxypyrazines, antioxidants, thiols, flavonols and anthocyanins.

Optimising AA concentrations in grapes is therefore important to both viticulturists and winemakers. Manipulation of grapevine nutrition and viticultural management can directly influence grape berry biochemical composition. Despite the importance of AAs, their regulation and accumulation in the grape berry is poorly understood and usually inferred through research in other species.

At Lincoln University, we have investigated changes in Sauvignon blanc grape AA composition throughout development (Gregan et al., 2012). Removing leaves from around the fruiting zone significantly reduces total AAs in the berries/juice/must at harvest. Two AAs in particular, arginine and proline, accumulate to high levels and make up the highest proportion of total AA concentrations in the grape at harvest. Both of these AAs are significantly decreased in experiments with our leaf removal treatments. Proline accumulation is greatest post-veraison and it becomes one of the predominant AAs at harvest, whereas the accumulation of arginine begins pre-veraison and increases at a more consistent rate throughout development of the berry. Arginine is an important preferential nitrogen source for yeast in ferments, while proline is a non-YAN AA. Although many studies link proline accumulation in plants to environmental stresses, the correlation between proline levels and stress tolerance is not always apparent and many aspects of its biological functions are still very unclear.

To advance our understanding of the mechanisms involved in AA accumulation in Sauvignon blanc grapes, we have performed leaf removal experiments across two seasons collecting samples at multiple timepoints, with emphasis on increased sampling through veraison. We have quantified total AAs through development to define developmental stages where there is a “switch” in berries to assimilate (i.e. through glutamate and glutamine), accumulate, degrade or redistribute AAs. We have also investigated genetic regulation of nitrogen assimilation and amino acid metabolism. Using Nanostring nCounter technology has enabled us to simultaneously investigate multiple known and putative genes involved in AA assimilation (glutamine and glutamate) and proline/arginine metabolism. This will enable us to get a clear overview of expression profiles through development and identify key genes for further examination.

We will be presenting our initial results from these experiments and aim to provide insight to these findings that will be of interest to the industry. We hope an improved understanding will be of future benefit to viticulturists for canopy management and as the biochemical composition of grapes determines the finished product, to winemakers for optimising wine quality.